Teachings of fuel cells, fuel cell stacks, fuel cell stack assemblies, and heat exchanger systems, arrangements and methods are well known to one of ordinary skill in the art, and in particular include WO02/35628, WO03/07582, WO2004/089848, WO2005/078843, WO2006/079800, WO 2006/106334, WO 2007/085863, WO 2007/110587, WO 2008/001119, WO 2008/003976, WO2008/015461, WO2008/053213, WO2008/104760, WO2008/132493, WO2009/090419, WO2010/020797, and WO2010/061190, which are incorporated herein by reference in their entirety. Definitions of terms used herein can be found as necessary in the above publications. In particular, the present invention seeks to improve the systems and methods disclosed in WO2008/053213.
Operating hydrocarbon fuelled SOFC (solid oxide fuel cell) systems where the fuel cell stack operates in the 450-650 Deg C. range (intermediate-temperature solid oxide fuel cell; IT-SOFC), more particularly in the 520-620 Deg C. temperature range, results in a different set of technical problems being encountered and requires a different approach as compared to higher temperature SOFC technology such as YSZ (yttria-stabilised zirconia) based technologies which typically operate at temperatures >720 Deg C.
The lower fuel cell stack operating temperature does not lend itself to high levels of internal reforming of fuel and thus such systems typically require high levels of reforming prior to fuel reaching the fuel cell stack.
In such systems, steam reforming is used to convert a hydrocarbon fuel stream into a hydrogen-rich reformate stream which is fed to the fuel cell stack anode inlet. The reformer is typically operated in a temperature range of 620-750 Deg C. such that the output reformate is in the temperature range 500-750 Deg C., allowing reforming of over 80% of the hydrocarbon (such as natural gas). The reformate stream is then cooled to about 350-550 Deg C. for entry into the fuel cell stack at about 450 Deg C. The reformer is typically heated by the output of the tail-gas burner which combusts the fuel cell stack off-gases.
IT-SOFC stack cooling is achieved mainly through control of the oxidant flow over the cathode side of the fuel cell stack (i.e. to effect increased cooling, more air is blown over the cathode side of the fuel cell stack). This is different to other SOFC technologies where higher levels of internal reforming occur and where the resulting endothermic effect of the internal reforming reaction acts to absorb thermal energy released from the operating fuel cell.
To achieve the above high reformer temperature, the reformer is usually closely thermally coupled with the fuel cell stack tail-gas burner (which burns any remaining fuel in the anode off-gas in oxidant, typically by combusting with the hot cathode off-gas). In such an arrangement, the tail-gas burner and its hot exhaust gas are closely thermally coupled with the reformer by way of a heat exchanger such as a heat exchange surface. Typically, the reformer is arranged so that it is immediately adjacent to or in contact with the tail-gas burner in order that as much heat as possible is passed from the tail-gas burner to the reformer.
The present inventors have identified a number of technical limitations which affect current IT-SOFC fuel cell stack arrangements:
1. IT-SOFC Degradation Leads to a Significant Non-Linear Loss of Fuel Cell Stack Efficiency
During the life of a fuel cell, degradation in the fuel cell leads to a loss of electrical efficiency, and therefore an increased heat production for a given electrical power output. Controlling fuel cell stack operating temperature is critical for fuel cell stack operating performance. For a fuel cell system, the delivery of fuel cell stack cooling (in particular by pumps/blowers to the cathode side of the fuel cell) is a substantial system parasitic load (typically, the largest system parasitic load). As fuel cells degrade, this combination of loss of efficiency and increased parasitic load provides a disproportionate (i.e. a greater than linear, also referred to herein as a non-linear) reduction in efficiency at the system level.
Further, as the fuel cell stack provides the electrical power to provide fuel cell stack cooling, a positive feedback mechanism (i.e. a vicious cycle) is initiated by a loss of fuel cell efficiency, i.e. the fuel cell stack is less efficient and generates more heat for a given electrical output, and therefore needs more cooling which results in an increased power demand, requiring increased power generation, in turn resulting in further increase in heat generation requiring a further increase in cooling.
2. Close Thermal Coupling of the Reformer to the Tail-Gas Burner Results in Increased Fuel Cell Stack Cooling Load
Close thermal coupling of the (endothermic) fuel reformer to the tail-gas burner (TGB) means that the enthalpy of the fuel flow leaving the fuel reformer is a function of the total airflow to the fuel cell stack. With IT-SOFC degradation, the increased electrical resistance and thus increased fuel cell heat generation results in increasing reformer temperature and thus increasing hydrogen content in the reformed fuel, in turn increasing the fuel cell stack cooling load during fuel cell stack operation.
Without supplementary heat recuperation for the anode inlet gas between the between the reformer outlet and the fuel cell stack anode inlet, this increased thermal energy is transferred to the fuel cell stack as additional cooling load, which further increases gross power requirements and results in a further decrease in fuel cell system efficiency.
3. Carbon Monoxide Produced as a Product of Reformation Causes Carbon Drop-Out and Metal Dusting, Resulting in Degradation to the Fuel Cell Stack Anode Side
Carbon drop-out from reformed fuel has a significant negative effect upon fuel cell stack performance, particularly during extended use. As reformate containing carbon monoxide exits the reformer and passes to the IT-SOFC stack anode inlet, it typically undergoes a significant decrease in temperature due to the fact that reformers are usually operated at a high temperature in order to achieve a high level of reformation. As a result of that temperature decrease, the equilibrium between carbon monoxide and carbon dioxide shifts in favour of carbon dioxide—the Boudouard Reaction takes place, carbon monoxide is oxidized into carbon dioxide, and carbon precipitates, i.e. carbon drop-out occurs. This carbon drop-out is in the form of (i) particulate carbon, which can coat surfaces and block/restrict fluid flow paths, and (ii) metal dusting (“Corrosion by Carbon and Nitrogen: Metal Dusting, Carburisation and Nitridation”, edited by H. J. Grabke and M. Schütze, 2007, ISBN 9781845692322) where the carbon forms on the surface of exposed metal surfaces of components, resulting in metal being removed from the body of the component over time with a corresponding negative impact on the component specification.
These limitations are typically not seen in higher temperature fuel cell systems because a degree of internal reforming is capable and indeed desirable to reduce blower parasitic loads and any external reformate is inevitably much closer to fuel cell stack operating temperature and thus does not require cooling through the Boudouard Reaction temperature range.
The present invention seeks to address, overcome or mitigate at least one of the prior art disadvantages.